Full-face overburden drilling systems that use a crown bit and a driver drill bit are well known in the art. These two parts are typically used in the first step of a two step drilling procedure, wherein the first step involves drilling through the overburden material, which may include earth, sand, clay, gravel and boulders for example, in order to reach harder ground material commonly referred to as bedrock. The second step involves sub-casing drilling through the bedrock with a conventional drilling method.
In existing full-face overburden drilling systems, in order to accomplish the first step, the driver drill bit and the crown bit are releasably connected in a locking engagement and rotate together in order to drill through the overburden material. The crown bit, which defines a hollow passageway for receiving the driver drill bit, is connected to the base of a casing. The crown bit can be connected to the casing with bolts, threads or weldings, for example. Therefore, in use, as the driver drill bit and the crown bit drill into the ground, the casing, which has been connected to the crown bit, is pulled into the hole with the crown bit. The casing acts to prevent the hole from collapsing in upon itself. Once the driver drill bit and the crown bit have passed through the majority of the overburden material, to the depth at which the bedrock begins, the driver drill bit is unlocked from the crown bit and pulled out of the hole through the casing. The casing and crown bit remain in the hole. A standard drill bit is then inserted into the hole through the casing and through the crown bit, at which point the standard drill bit is used for sub-case drilling in the bedrock.
Typically, in order to allow for the crown bit and the driver drill bit to be locked together during drilling, the two components are provided with a projection-recess assembly. More specifically, the crown bit includes at least three projections that extend into its hollow passageway and that are adapted to engage with mating recesses provided on the driver drill bit.
A deficiency with this type of arrangement is that the projections of the crown bit limit the size of the drill bit that can be used, since the drill bit must pass through the passageway of the crown bit. As such, the maximum diameter of the drill bit for sub-casing drilling is determined by the diameter of the portion of the hollow passageway of the crown bit that includes the projections. As such, the drill bits being used with existing full-faced overburden drilling systems are smaller in size than what could theoretically pass through the casing in the drilled hole.
In down the hole drilling, the drilling components, meaning the driver drill bit, and later on the drill bit for drilling through the bedrock, are connected to a hammer. It is the hammer that provides the drilling components with the necessary force and vibration to drill into the ground. When a smaller drill bit is used for drilling through the bedrock because the hollow passageway of the crown bit limits its size, a smaller hammer must be used. Unfortunately, a smaller hammer imparts less force and vibration to the hammer and therefore takes longer to do the same job as a larger drill bit that could use a larger hammer. As such, existing full-face overburden drilling systems are operating at a reduced efficiency.
Against this backdrop, it can be seen that there is a need in the industry for an overburden drilling system that alleviates, at least in part, the deficiencies associated with the prior art full-faced overburden drilling systems.